1. Field of the Invention
This invention relates to communication equipment and more particularly to a reference signal generator and correlator for receiving minimum shift keyed data.
2. Description of the Prior Art
Communication equipment may use minimum shift key (MSK) modulation wherein a spreading function, such as a pseudo-random noise, is used to provide a plus or minus 90.degree. phase shift to the carrier at the MSK modulation rate (chipping rate). Binary phase shift keyed (BPSK) and quaternary phase shift keyed (QPSK) modulations also provide spreading functions, such as pseudo-random noise, by shifting the phase 0.degree. or 180.degree. or 0.degree., 90.degree., 180.degree., 270.degree. of the carrier signal in response to the spreading function code. Pseudo-random noise spreading functions are used for secure communications using a rapidly changing pseudo-random noise code which may change after one second or less. The rapidly changing code provides a low probability of intercept (LPI) to the message, inhibiting exploitation, spoofing and jamming. MSK modulation is desired because it permits more information for a given bandwidth. MSK requires more complex hardware, however, since the modulating signal is complex having an in-phase and quadrature component which generally limits the speed of operation.
Presently pseudo-random noise spreading function codes with BPSK modulation and moderately high chipping rates are correlated in surface acoustic wave (SAW) convolvers or programmable surface acoustic wave correlators at intermediate frequencies. The surface acoustic wave convolvers and programmable surface acoustic wave correlators are very expensive in small quantities and may not be practical for MSK modulation.
Another technique for receiving BPSK modulation involves translating the received signals to baseband and processing them digitally for both in-phase (I) and quadrature (Q) components of the signal. To achieve significant dynamic range, such as 60 db or more, an analog-to-digital converter with ten bit resolution or more is necessary. Both the analog-to-digital converter and digital processing for ten bits resolution in a BPSK correlator is complex, expensive, slow, bulky and consumes a lot of power. For receiving MSK modulation, the circuitry is doubled because of the complex number samples of the correlation reference.
Presently, a semiconductor device has been developed for receiving BPSK modulation. The device is manufactured by Reticon Corporation, a subsidiary of EG & G, Inc. located in Wellesley, Mass., bearing part number R5401, which performs correlation between a programmable binary function and an analog input signal. The programmable binary function is stored in a 64 sample static binary register. Each binary sample controls a switch for coupling data coupled to a corresponding terminal for a 64 sample dynamic analog register to a plus or minus sum bus. The analog signal is sampled and loaded into the 64 sample dynamic register. The plus and minus sum bus provide a correlation output with respect to the 64 sample static binary register to the 64 sample dynamic analog register. The device R5401, is applicable to radars using barker or pseudo-random codes and to spread spectrum for anti-jam communications. For BPSK modulation, 64 samples (in-phase) are loaded into the analog register. Sixty-four samples of the binary reference signal at baseband are loaded into the binary register. Each bit of the binary reference controls a single pole, double throw (SPDT) switch with the single pole coupled to the respective tap of the analog register. If the reference bit is a 1, the analog signal at the respective tap is switched to a plus bus; if a zero, the analog signal at the respective tap is switched to a negative bus. The two buses sum the analog signal inputs and are coupled to respective inputs of a difference amplifier. The output of the difference amplifier is then the correlated BPSK signal at the reference phase of the baseband input signal. The quadrature phase samples (Q) can be obtained in a similar way. By taking the magnitude such as by the square root of the sum of the squares of the in-phase and quadrature signal, the correlated signal may be obtained.
A device such as R5401 works for BPSK since the reference signal is a binary code spreading function, with constant amplitude bits. The samples of this waveform must effectively weight the analog signal at each respective tap by multiplying the weight and the analog signal together. The multiplication is accomplished by simply switching the analog signals at the taps to either the plus or minus sum buses. While the BPSK reference signal is binary and has constant amplitude bits, the MSK reference signal is neither a replica of binary code, real only, or of constant amplitude.
In a publication of F. J. Kelly, "Sampled-Data Receiver Design and Performance for BPSK and MSK Spreading Modulations", Technical Report 550, Lincoln Laboratory, Massachusetts Institute of Technology, Jan. 19, 1981, an analysis of receiver performance is described in terms of SNR loss due to sampling. Table IV at page 33 indicates the average signal-to-noise ratio loss when sampling at 1, 2 and 4 times the chip rate with a filter matched to the MSK pulse.
It is therefore desirable to generate a minimum shift keyed reference signal which is constant amplitude and binary.
It is further desirable to provide a minimum shift keyed reference signal which may utilize a correlation device such as R5401 manufactured by the Reticon Corporation located in Wellesley, Mass.
It is further desirable to generate a MSK reference signal at baseband from the code and sample the complex MSK signal received so that all weights are of equal magnitude, plus and minus signals.
It is further desirable to use a MSK reference signal at baseband where all weights are of equal magnitude, plus and minus, using a plurality of correlation devices for receiving MSK signals.
It is further desirable to provide a correlator for receiving minimum shift keyed data utilizing a signal generator for generating a MSK reference signal from a predetermined code where all weights are of equal magnitude, plus and minus signals.